Contactor and trip circuit therefor

ABSTRACT

A contactor comprises a switching circuit for switching a load between one or more electrical supplies, a current detector for detecting the current flowing through the load, a trip circuit for tripping the current after an interval of time which depends upon how much the current flowing through the load exceeds a predetermined amount, and a pre-trip circuit for providing an indication during said time interval that the current exceeds said predetermined amount for providing ‘pre-trip’ or ‘near-trip’ information.

[0001] This invention relates to a contactor and trip circuit therefor.The contactor is operative for switching one or more high current loadsbetween one or more electrical sources, for example an AC generator andDC batteries to ensure a constant electrical supply. A trip circuit isprovided having an overload trip capability. The contactor, which may bein the form of a solid state contactor, may be used for providinginterrupt free direct electrical power of, for example 150 Amps, toindividual DC busses on an aircraft.

[0002] Conventional contactors may be provided with a trip capabilityfor terminating the supply of current to a load when it exceeds anominal operating level by a predetermined amount.

[0003] It is an aim of the present invention to provide a contactorwhich is capable of providing an indication of near trip events, thatis, instances where the load exceeds the nominal operating level but notby enough or for long enough to trip the supply. It is therefore an aimof the present invention to provide a contactor having a ‘pre-trip’history capability.

[0004] According to the present invention there is provided a contactorcomprising a switching circuit for switching a load between electricalsupplies, a current detector for detecting the current flowing throughthe load, a trip circuit for tripping the current after an interval oftime which depends upon how much the current flowing through the loadexceeds a predetermined amount, and a pre-trip circuit for providing anindication during said time interval that the current exceeds saidpredetermined amount for providing ‘pre-trip’ or ‘near-trip’information.

[0005] The predetermined amount may be 10% above the maximum currentwhich the contactor can carry without overload.

[0006] The pre-trip or near-trip information may be supplied to amonitoring computer for monitoring pre-trip activity. Embodiments of theinvention are advantageous in that they permit monitoring of eventswhich give rise to an overload current which are insufficient induration or quantum to cause the contactor to trip but whichnevertheless are indicative of the state of an overload condition.

[0007] In a preferred embodiment, the trip circuit includes a functiongenerator for converting the sensed current flow to a signal currentproportional to the required interval of time. The function generatorcircuit preferably has a response time characteristic which variesdepending on how much the current exceeds the predetermined amount. Theresponse time characteristic is preferably such that the time from whenthe current breaches the predetermined amount to occurrence of theeventual trip decreases as excess of the current over the predeterminedamount increases. The time characteristic preferably resembles the I²Trelationship of a bimetallic strip in which the heating characteristicof the strip varies with the square of the current I flowing through it(temperature is proportional to the square of the current multiplied bythe resistance), where T is the trip time.

[0008] The trip circuit may be configured to generate a digital-typeoutput representing respective ones of a plurality of trip timeintervals each corresponding to a sub-range of 1 ²T overloads. Thefunction generator circuit may be provided with a pulse generator forgenerating pulses at a frequency which depends on the range of currentoverload. A counter is provided for generating a trip signal when a setnumber of pulses have been counted, the time interval being a functionof the pulse frequency and the set number. The set number and frequencycan be varied depending upon the required trip time interval for a givencurrent overload. Conceivably, alternative parameters may be used togovern the time intervals, such as pulse amplitude and/or width inaddition to or instead of frequency.

[0009] The function generator may include a current indicator forindicating the presence of current flowing through the load at levelsbelow the nominal value. For example, the indicating means may indicatecurrent at 10% of a 150 Amp nominal rating. An absolute overload currentdetector may additionally be provided for detecting substantial overcurrents arising from, for example, short-circuits and the like. Theabsolute overload current detector can be operative to cause thecontactor to trip instantly. The detector may be configured to respondto overload currents in the order of 1000% of the nominal rating.

[0010] The current detector preferably includes two Hall effecttransducers which sense the current in a power stage to which thecontactor is coupled. One of the Hall effect transducers has arelatively high sensitivity for sensing low level currents and the othera lower sensitivity for high currents. Alternatively, a single resistivesensor may be used. Hall effect transducers are chosen with ranges toimprove measurement accuracy around the trip threshold. Additionalcalibration may be necessary for alternative sensors.

[0011] The contactor embodying the present invention is provided with atrip recovery delay operative for holding the tripped state for apredetermined period of time to allow for cooling. After the triprecovery delay, the tripped contactor may be manually overridden oroperated under control of an external computer.

[0012] The predetermined period of time may vary in dependence upon theextent of the overload which gave rise to the trip.

[0013] According to the present invention, there is also provided a tripcircuit for a contactor, the trip circuit comprising means for trippinga supply of current after an interval of time which depends upon howmuch the current flowing through a load exceeds a predetermined amount,and a pre-trip circuit for providing an indication during said timeinterval that the current exceeds said predetermined amount forproviding ‘pre-trip’ or ‘near-trip’ information.

[0014] Embodiments of the invention have the advantage that they providefor a quantitative monitoring of pre-trip events for diagnosticpurposes.

[0015] The invention will now be further described by way of example,with reference to the accompanying drawings, in which:

[0016]FIG. 1 is a block diagram of a trip circuit for a contactorembodying the present invention;

[0017]FIG. 2 is a graph showing how trip times may vary in relation tosensed current in the trip circuit embodying the present invention inthe case where the function generator circuit is set to have thecharacteristic of a thermal circuit breaker;

[0018]FIG. 3 shows the oscillator output for four different currentlevels;

[0019]FIG. 4 is a circuit diagram of a function generator which may beemployed in a trip circuit embodying the present invention; and

[0020]FIG. 5 is a circuit diagram of a trip time generator which may beemployed in the trip circuit embodying the present invention.

[0021] In FIG. 1 a trip circuit and pre-trip circuit embodying thepresent invention is shown in block schematic form. In this embodiment,a pair of Hall effect transducers HL and HH generate outputs indicativeof the current passing through a load to which the contactor is coupled.The two Hall effect transducers HL and HH are scaled for high accuracyat low current levels and reduced accuracy at high current levelsrespectively. For example, in the case where the trip current is definedas 150 Amps, Hall effect transducer HL is arranged for sensing nominally0% to 250% of the trip current. Hall effect transducer HH is set forsensing nominally 0% to 1000% of the trip current. This arrangement isused to achieve the required accuracy over the extended operationalrange. Alternatively, in place of the Hall effect transducers, a singlesensor design may be adopted through use of a resistive sensing means.This would combine the function generator inputs but retain all thefunctional blocks.

[0022] The trip circuit includes a function generator 1 which includes acurrent indicator 2 for receiving the output from the low current Halleffect transducer HL and provides an output I to indicate the presenceof a current on the load even though this may be only 10% of the nominalvalue. This output is supplied to a database and display D whichmonitors the detection of the load on the current and displaysappropriate information. The function generator 1 also includes a lowcurrent trip circuit 3 which also receives the output of the low currentHall effect transducer HL and generates an output IO at a first level,this being fed to an oscillator 4 and reset 5. The load current tripcircuit 3 produces the output IO when the magnitude of the currentsensed in the load by the low current Hall effect transducer HL exceedsthe predetermined amount by a relatively low level. In this case, thecircuit does not trip immediately but after a relatively long timeinterval as described in more detail below. In the event that thecurrent sensed on the load exceeds the predetermined amount moresignificantly, the high accuracy Hall effect transducer HH is sufficientto cause a high current trip circuit 6 to generate a higher level outputIO than the one generated by the low current trip circuit 3. These twooutputs are summed and both are also fed to the oscillator 4 and thereset 5.

[0023] The function generator 1 is provided with an overload trip 7which is operative for generating a trip signal OT fed directly to atrip switch 8 in the event that the load sensed by the Hall effecttransducer HH senses a catastrophic overload of, for example, 1000% ofthe nominal value. The trip switch 8 may be implemented by one or morefield effect transistors. Each of the low and high current trip circuits3 and 6 can provide various levels of output signal IO depending on themagnitude of the current sensed by its respective Hall effect transducerHL, HH as will be described in more detail below with reference to FIGS.4 and 5. The output signal IO can therefore have a range of differentvalues depending on the current level sensed by the Hall effecttransducers. Given that the low and high current trip circuits 3 and 6are configured to generate an output when the sensed current flowingthrough the contactor exceeds a predetermined amount, each level ofoutput IO will correspond to a different time interval to establish thetrip.

[0024] The oscillator 4 is configured to generate a range of pulsetrains, the frequency of which depend on the level of the signal IOreceived by the oscillator 4. The trip circuit is provided with acounter 9 operative for counting the pulses received from the oscillator4. When a preset number of pulses is counted, the counter 9 generates atrip signal T which is fed to the trip switch 8 for tripping the supplyof current to the load. FIG. 3 shows the oscillator output for fourdifferent current levels. When the overload current sensed by Halleffect transducers is low, the output IO corresponds to pulse trains ior ii of FIG. 3. In this case the preset number of counts will takelonger to reach than it will if the high current trip circuit 6generates a relatively high level output IO thereby causing theoscillator 4 to generate pulse trains with higher frequencies asillustrated by the trains iii and iv of FIG. 3.

[0025] The counter 9 is operative for generating a pre-trip signal PTeach time a pulse is counted. Consequently, the database and display Dis informed of the existence of an overload while the counter 9 iscounting its way to the preset number of counts which gives rise togeneration of the trip signal T. The number and frequency of thepre-trip signals PT supplied to the database and display D will beindicative of the pre-trip overload condition. This information isstored in the database and can be used for diagnostic purposes.

[0026] When the Hall effect transducers HL, HH no longer sense anoverload, the output IO will fall accordingly. The input to the reset 5will therefore cross its threshold giving rise to generation of a resetsignal for resetting the counter 9 so preventing pre-trip eventsaccumulating over a long period and resulting in a spurious trip. Ifhowever a trip does occur, before the trip is reset, it is desirable forthe trip circuit to be maintained in a tripped state for a predeterminedperiod of time to allow for cooling of circuit components which havebeen heated by the overload. A trip recovery delay 10 is provided togenerate a trip recovery signal when the counter 9 has generated a tripsignal T. The trip recovery signal TR is supplied to trip circuit 3 ofthe function generator 1 for holding the output IO active for thepredetermined period of time. In the event that a catastrophic overloadcurrent is detected, giving rise to generation of an overload tripsignal OT, an overload trip recovery delay 11 generates an overload triprecovery signal XTR which is supplied to the overload trip 7 to maintainthe generation of the overload trip signal OT for a longer predeterminedperiod of time.

[0027]FIG. 4 shows more detailed circuit components which may be adoptedin the function generator 1. The purpose of the function generator, withthe timer circuit, is to obtain a time to trip characteristic similar tothat shown in FIG. 2. This function generator circuit 1 generates asignal IO which is proportional to the square of the current detected bythe Hall transducers HL and HM. The low current portion of the functiongenerator circuit 3 comprises a pair of operational amplifiers 12, 13which receive the output from the low current Hall effect transducer HL.Respective outputs of the operational amplifiers 12, 13 are fed back totheir non-inverting inputs via resistors R1 and R2 respectively so thata current proportional to the voltage of the Hall effect transducer HLis generated over a predefined range. The output of the Hall effecttransducer HL is fed to the non-inverting input of the operationalamplifiers 12, 13 via resistors R4 and R5 respectively. The positiveinputs of the operational amplifiers 12 and 13 are derived from apotential divider formed from resistor chain R6 to R10. The values ofthe components are chosen so that relatively low current overload valuesare divided into two sub-ranges. When the Hall effect transducer HLvoltage corresponds to a load current within the lowest sub-range, theoutput of operational amplifier begins to rise from zero and havingcrossed the base emitter threshold of TI will start to produce a currentIO proportional to the rising voltage. When the voltage of the Halleffect transducer HL is within the next sub-range, the output ofoperational amplifier 13 begins to rise and a similar but largerproportional current is produced. This current is added to the existingIO which has by now reached its limiting value. The difference incurrent values produced by T2 over T1 is set by the relative values ofR11 and R12. The high current circuit 6 has a similar arrangement ofoperational amplifiers 14 and 15. The positive inputs of these arecoupled to a voltage divider made up of resistor chain R13 to R16. Thevoltage output of the Hall effect transducer HH is coupled to respectivenon-inverting inputs of the operational amplifiers 14 and 15 viarespective resistors R17 and R18. The output of the operationalamplifiers 14 and 15 are respectively fed back to the non-invertinginput via resistors R19 and R20. The values of resistors R17 to R20 arechosen so that the output of operational amplifier 14 begins to risewhen the voltage at the Hall effect transducer HH falls within a thirdsub-range and that output of the operational amplifier 15 begins to risewhen the voltage output of HH falls within a fourth sub-range. As eachamplifier output rises, it produces a further range of proportionalcurrents in T3 and T4 respectively. Each range of current being set bythe values of R21 and R22 respectively. These further currents aresummed to the signal IO. If a single sensing element is used, thefunctionality remains the same, but the inputs HL and HH are combined.

[0028] The result of the summed current, that is IO, is to dischargecapacitor C1 towards ground potential at a rate which is proportional tothe square of the level of detected overload current.

[0029]FIG. 4 also shows details of circuit components which may beadopted for the current indicator 2 and the overload trip 7. The currentindicator 2 comprises an operational amplifier 16 which generates afixed output i when the voltage received at its non-inverting inputexceeds a level such as to indicate the presence of a load. The overloadtrip 7 comprises a further operational amplifier 17 and is operative forgenerating an output OT when a catastrophic overload in the order of1000% of the nominal value is detected by the Hall effect transducer HH.

[0030] When the trip recovery signal TR is generated, R24 provides avoltage to the base of the transistor T2 so setting the rate of thenormal recovery clock. In the event of a catastrophic overload, givingrise to generation of the overload OT, resistor R27 performs the role ofR24 until OT is reset by input XTR.

[0031]FIG. 5 illustrates the main components which may be adopted in thepre-trip circuit embodying the invention. The oscillator 4 is connectedto the input IO that sinks current to ground as an I²T function asdiscussed above with reference to FIG. 4. The effect of the functiongenerator 1 is to discharge capacitor C1 towards ground potential at arate which is proportional to the square of the level of detectedoverload current. Capacitor C1, operational amplifier 18 and associatedresistors form a relaxation oscillator circuit 4. The frequency of thisoscillator is proportional to the current discharged from the capacitorC1 by function generator 4. With no current overload, the frequency ofthe oscillator is zero using as detected current 10. The pulse trainfrom oscillator 4 is coupled to the clock CLK input of a counter 19.Every time a pulse is received at the clock CLK terminal, a pre-tripsignal is generated and supplied to the database and display D formonitoring. After a pre-set number of counts, the clock generator 19generates an output which is supplied to a second clock generator 20which in turn gives rise to generation of a trip signal T which issupplied to the trip switch 8. The time taken to generate the tripsignal T is therefore dependent on the frequency of the pulse trains ito iv which in turn depends on which sub-range the overload currentcorresponds to. In the event that the overload current ceases, thetransistors T1 through T4 of FIG. 4 would switch off but T2 remainsactive by the action of R24 and R27 until, in order to allow for coolingof components which have been heated by the overload current, thecounters 19 and 20 complete a full cycle. Once the counters havecomplete the fill cycle, line TR goes low, T2 is switched off andoscillator 4 stops. The reset circuit then operates as the voltageacross C1 in oscillator 4 decays towards rail and counters 19 and 20 arefully reset. Only when TR has gone low is it possible to either manuallyor under computer control, reset the trip switch 8 and attempt torecover the load function.

1. A contactor comprising a switching circuit for switching a loadbetween one or more electrical supplies, a current detector fordetecting the current flowing through the load, a trip circuit fortripping the current after an interval of time which depends upon howmuch the current flowing through the load exceeds a predeterminedamount, and a pre-trip circuit for providing an indication during saidtime interval that the current exceeds said predetermined amount forproviding ‘pre-trip’ or ‘near-trip’ information.
 2. A contactoraccording to claim 1 , wherein the pre-trip or near-trip information issupplied to a monitoring computer for monitoring pre-trip activity.
 3. Acontactor according to claim 1 , wherein the trip circuit includes afunction generator for converting the sensed current flow to a signalcurrent proportional to the interval of time.
 4. A contactor accordingto claim 3 , wherein the function generator circuit has a response timecharacteristic which varies depending on how much the current exceedsthe predetermined amount.
 5. A contactor according to claim 4 , whereinthe response time characteristic is such that the time from when thecurrent breaches the predetermined amount to occurrence of the eventualtrip decreases as excess of the current over the predetermined amountincreases.
 6. A contactor according to claim 4 , wherein the timecharacteristic resembles the I²T relationship of a bimetallic strip inwhich the heating characteristic of the strip varies with the square ofthe current I flowing through it, where T is the trip time.
 7. Acontactor according to claim 6 , wherein the trip circuit is configuredto generate a digital-type output representing respective ones of aplurality of trip time intervals each corresponding to a sub-range ofI²T overloads.
 8. A contactor according to claim 3 , wherein thefunction generator circuit is provided with a pulse generator forgenerating pulses at a frequency which depends on the range of currentoverload.
 9. A contactor according to claim 8 , wherein a counter isprovided for generating a trip signal when a set number of pulses havebeen counted, the time interval being a function of the pulse frequencyand the set number.
 10. A contactor according to claim 9 , wherein theset number and frequency can be variable depending upon the trip timeinterval for a given current overload.
 11. A contactor according toclaim 9 , wherein the time intervals are governed by pulse amplitudeand/or width in addition to or instead of frequency.
 12. A contactoraccording claim 3 , wherein the function generator includes a currentindicator for indicating the presence of current flowing through theload at levels below the nominal value.
 13. A contactor according toclaim 1 , wherein an absolute overload current detector is additionallyprovided for detecting substantial over currents.
 14. A contactoraccording to claim 13 , wherein the absolute overload current detectoris operative to cause the contactor to trip instantly.
 15. A contactoraccording to claim 13 , wherein the current detector includes at leasttwo Hall effect transducers which sense the current in a power stage towhich the contactor is coupled, a first one of the Hall effecttransducers having a relatively high sensitivity for sensing low levelcurrents and a second one of the Hall effect transducers having a lowersensitivity for high currents.
 16. A contactor according to claim 1 ,comprising a trip recovery delay operative for holding the tripped statefor a predetermined period of time to allow for cooling.
 17. A contactoraccording to claim 16 , wherein the predetermined period of time mayvary in dependence upon the extent of the overload which gave rise tothe trip.
 18. A trip circuit for a contactor, the trip circuitcomprising means for tripping a supply of current after an interval oftime which depends upon how much the current flowing through a loadexceeds a predetermined amount, and a pre-trip circuit for providing anindication during said time interval that the current exceeds saidpredetermined amount for providing ‘pre-trip’ or ‘near-trip’information.
 19. A contactor comprising a switching circuit forswitching a load between one or more electrical supplies, a currentdetector for detecting the current flowing through the load, a tripcircuit for tripping the current after an interval of time which dependsupon how much the current flowing through the load exceeds apredetermined amount, and a pre-trip circuit for providing an indicationduring said time interval that the current exceeds said predeterminedamount for providing ‘pre-trip’ or ‘near-trip’ information, wherein thecontactor is operative for permitting the monitoring of events whichgive rise to an overload current which are insufficient in duration orquantum to cause the contactor to trip but which nevertheless areindicative of the state of an overload condition.